As is known, display devices using OLEDs comprise an emission region formed from a matrix of pixels, each pixel typically consisting of a plurality of differently colored subpixels (RGB: red, green and blue in general), and an electrical connection region arranged adjacent this active region. Each pixel of this OLED matrix usually incorporates a multilayer, light-emitting structure comprising an organic film interposed between two, top and bottom, electrodes that serve as anode and as cathode and one of which is transparent or semitransparent to the light emitted whereas the other is generally reflecting.
To manufacture these screens, organic layers are deposited on each subpixel (one type of layer or multilayer stack per color) by means of a shadow mask. The minimum dimension of the apertures of this shadow mask defines therefore a minimum size for these subpixels. This minimum size for each subpixel may also be imposed by the dimensions of the addressing circuit used to supply electrical power to each subpixel via the aforementioned connection region. It has therefore been sought to increase the resolution of screens by exploiting one of these two parameters i.e. the size of the apertures of the shadow mask or, if it is the limiting factor, the size of the addressing circuit.
Other ways of improving the resolution of such screens have been investigated in the past, consisting in producing stacks of OLED units. In this type of screen, certain subpixels and their corresponding emitting structures are located in what is called an external OLED unit, adjacent the emission side, whereas others are located in what is called an internal OLED unit, adjacent the substrate, an electrode of the pixel thus formed possibly being common to two superposed units.
It has moreover been sought, in these stacks, to minimize the current density flowing in the subpixel that is the most sensitive to aging—that which has the shortest wavelength, typically the blue, in the case of RGB—by increasing its emission area at the expense of that of the other subpixels, so as to increase the lifetime of this critical subpixel and therefore of the whole device.
It is possible, for example, to mention document U.S. Pat. No. 6,747,618, which presents, in its FIG. 8, such a stack, with the two, red and green, subpixels located in the external unit, and the blue subpixel located in the internal unit with an emission area greater than the sum of the areas of the red and green subpixels. It is also possible to mention document U.S. Pat. No. 7,250,722 which describes a stack in which the red and blue subpixels are located in the same internal or external unit and the green subpixel is located in the other unit, while still ensuring that the emission area of the blue subpixel is greater than that of the other subpixels so as to increase its lifetime.
A major drawback of the stacks of OLED units presented in these documents is that the increase in lifetime that they procure, in each pixel, for the critical (typically blue) subpixel, by maximizing the area of its emitting structure, does not allow the resolution of the screen to be optimized, the resolution remaining limited by the shadow mask used for the deposition of the subpixels of smaller dimensions.
Another drawback of known stacks, where the critical (e.g. blue) subpixels are located in the internal OLED unit, is that the photons that they emit are reabsorbed by the emitting structures of the other subpixels in the external OLED unit, resulting in a loss of flux for these photons on the emission side of the screen.